This invention relates to a hydraulic torque impulse generator of the kind having a motor rotated drive member formed with a fluid chamber, an impulse receiving output member coaxial with the drive member and extending into the fluid chamber, a hydraulic pressure peak generating means in the fluid chamber for producing torque impulses at relative rotation between the drive member and the output member, and a variable volume fluid accumulator chamber located in the drive member and connected to the fluid chamber for compensating for occurring volume changes in the hydraulic fluid.
An impulse generator of this type is previously described in U.S. Pat. No. 4,789,373. In this prior art impulse generator, an annular accumulator chamber is located in a coaxial relationship with the output shaft and communicating with the fluid chamber via the clearance seal arround the output shaft. An annular piston is reciprocable in this chamber and is provided with 0-rings to seal off the accumulator chamber, both at its outer circumference and at its coaxial bore surrounding the output shaft. This known fluid volume compensating device is rather bulky and difficult to get completely fluid tight due to the movable seals.
In EP 0 309 625, there is disclosed another impulse generator of the type stated above, wherein a volume compensating piston-cylinder device is provided laterally but in parallel with the rotation axis of the output shaft. Also in this impulse generator, the compensation piston is fitted with 0-ring seals which are difficult to get completely fluid tight. This means that after some period of operation, hydraulic fluid has leaked past the piston seals and filled up the spaces on both sides of the piston with fluid, thereby making the volume compensating device inoperable.
Still another impulse generator of this type is described in U.S. Pat. No. 4,533,337. The volume compensating device of this impulse generator comprises an annular expansion chamber which is located in the rear end wall of the drive member and which is filled with a foamed plastic material. At expansion of the hydraulic fluid, the excessive fluid enters the expansion chamber and compresses the elastic material. The elasticity of this material is based on the fact that foamed material comprises a great number of gas filled closed cells or bubbles. A serious problem inherent in this device is the poor resistance of foamed plastic material against destructive influence of hydraulic fluid. The service life of this type of volume compensating devices is rather short, since the very thin walls of the closed cells do not withstand for very long the aggressive environment formed by the hydraulic fluid. After collapse of the closed cells the foamed material will just get soaked by the hydraulic fluid and will not provide any elasticity.